[docs] add information about 'Item' API to Connection docs

Change-Id: I4b7d7075fd9a3fc909077809ab1f1c0b35ce11a1
Reviewed-on: http://review.couchbase.org/28229
Tested-by: Mark Nunberg <mnunberg@haskalah.org>
Reviewed-by: Volker Mische <volker.mische@gmail.com>